Traditional operating systems (OS) rely on separate interactive sessions for each user. They may allow multiple users to switch in and out of an operating system and application experiences, but user interactions and integration with services and features is typically limited to one user at a time.
Enabling parallel application functionality on a traditional operating system typically requires an application developer to conceive their own user model that is managed in the application layer. This fundamentally limits the user-related customization that can be achieved since there is no support for multiple users in a same application session.